Bend Pivot Center of Rotation
The equation that describes the Y coordinate of the bending pivot could be:



Where:


R is the wanted radius (in Blender unit)
BBsize.x is the bounding box size (in Blender units) on the X axis of the undeformed object
Theta is the angle of deformation (in radians)


If I would like to know, for example, where to put the bending pivot for the default plane (BBsize.z = 2 units) bended by one radiant (57,2958°) around a  circumference with a 3 units Radius, I'll get:

3-2/1= 1 units


If I want to increase the Radius to 5, I must move the pivot point in Y =

5-2/1 = 3 units




Changing the deformation angle to 45° (0,785398 rad) would mean changing the coordinate to:

5-2/0.785398 = 2.45352038075 units


Final result for the latest set of values:



Note that, as in the provided result we are using the world origin as the ring center, we can get rid from the dipendency of R, so the location of the Bending pivot is:

5-(5-2/0.785398) = 2/0.785398 = 2.54647961925 units


Here's a file with a bunch of drivers that change the deformation angle and the bending pivot accordling to the number of elements composing the ring that you can examine: